1. Field of the Invention
The present invention is related to an organic light-emitting laminate for use in an organic luminescent device, and an organic electroluminescent device, especially an organic electrophosphorescent device, containing the organic light-emitting laminate.
2. Description of the Prior Art
Recently, with rapid development of electronic products, the conventional CRT (cathode ray tube) displays cannot meet the demand of the market, and the demand for flat panel displays, especially those with lightness, thinness, power saving and high definition, is increasing. Being such circumstances, various organic electroluminescent devices, which have advantages of self light emission, no viewing angle limitation, power saving, easy fabrication and no need for backlight, have been developed.
The organic electroluminescent device emits light by the following mechanism. When an external electric field is applied to an organic semi-conducting film, electrons and holes are injected respectively from a cathode and an anode into the light-emitting layer of the film and are transported across the layer. As the electrons meet the holes in the light-emitting layer, they are recombined to form excitons, which can transfer energy to the light-emitting molecules in the light-emitting layer to make the light-emitting molecules excited. The excited light-emitting molecules will release energy in a form of light when they return to their ground state. Therefore, in brief, the organic electroluminescent device is a device for generating light from electricity.
The organic electroluminescent device generally comprises an anode (such as indium tin oxide (ITO)), a hole-transporting layer vapor-deposited on the anode, an organic light-emitting layer vapor-deposited on the hole-transporting layer, an electron-transporting layer vapor-deposited on the organic light-emitting layer, and a metal layer as cathode formed on the electron-transporting layer.
The color of the light emitted by the organic electroluminescent device depends on the organic fluorescent material(s) contained in the light-emitting layer. In general, in order to elevate the recombination efficiency of carriers and the luminous efficiency of the device, a guest light-emitting material with higher luminous efficiency is doped into the host light-emitting material. One example of the guest light-emitting material is an organic transition metal complex, which can emit phosphorescence due to “the heavy atom effect” of transition metal when it returns to its ground state from its triplet state. An organic electroluminescent device comprising a host light-emitting material and an organic transition metal complex as guest light-emitting material, also called “organic electrophosphorescent device”, can utilize all of excitons including those in singlet state and those in triplet state; therefore, its internal quantum efficiency can reach about 100%.
In the organic electrophosphorescent device, the host light-emitting material should have a triplet energy level higher than that of the guest light-emitting material (phosphorescence-emitting material), to avoid energy back transfer to the host light-emitting material (Ref. Appl. Phys. Lett., 83, 569 (2003)). Since singlet energy level is usually higher than triplet energy level for an organic light-emitting material, the host light-emitting materials with high triplet energy level surely has higher singlet energy level, which results in widening of the energy gap between the light-emitting layer and its adjacent layers. Widening of the energy gap may make to the barrier for inject charges into the light-emitting layer; as a result, higher operational voltage may be needed.
In other words, for an organic electroluminescent device that can emit white phosphorescence, if a light-emitting layer comprising a host light-emitting material with a wide energy gap is used, luminous efficiency will be increased, however, operation voltage will be undesirably elevated.
Therefore, it is desired to develop an organic electroluminescent device, especially an organic electrophosphorescent device, which has higher luminous efficiency and lower operational voltage.